(Not Applicable)
Synthetic aperture radar (SAR) was first developed in the 1950""s and utilizes an antenna looking to the side from a vehicle moving along a path to produce high resolution two-dimensional radar images. Because scanning is provided by the movement of the antenna, a small SAR antenna has the ability to have the resolution of a very large antenna. SAR systems currently make two dimensional images having resolution on the order of less than a foot.
Interferometric SAR (IFSAR) extends traditional two dimensional SAR processing to three dimensions by utilizing the phase difference between two SAR images taken from different elevation positions to determine an angle of arrival for each pixal in the scene. This angle, and the two-dimensional location information in the traditional SAR image, are transfomed into geographic coordinates, including height information, if the position and motion parameters of the antennas are known accurately.
IFSAR traditionally comprises a pair of antennas which are rigidly mounted with respect to each other and move along a path and collect at least two channels of data from many positions along the path. The typical IFSAR image is formed by using range and azimuth information to compute a complex SAR image for each antenna, and then cross correlating these images on a pixel-by-pixel basis to form the IFSAR image. The phase difference between corresponding pixels is attributed to target height, and a phase-unwrapping process may be used to resolve ambiguities.
For a quick review, let the IFSAR collection geometry be defined by FIG. 1. For a typical application, the radar uses a Linear-FM chirp, stretch processing, and quadrature sampling, as is well known in the art. Neglecting residual video phase errors, the phase of the video signal from an echo of an ideal point target located at s can be modeled approximately as                                                         Φ              video                        ⁡                          (                              i                ,                n                ,                k                            )                                ≈                                                    {                "AutoRightMatch"                            ⁢                                                                    4                    ⁢                    π                                    c                                [                "AutoRightMatch"                            ⁢                              f                                  n                  ,                  k                                                      +                                                            B                                      eff                    ,                    n                    ,                    k                                                  ⁡                                  (                                      i                    I                                    )                                            ⁢                              "AutoLeftMatch"                ]                            ⁢                              (                                                      "LeftBracketingBar"                                          r                                              c                        ,                        n                        ,                        k                                                              "RightBracketingBar"                                    -                                      "LeftBracketingBar"                                                                  r                                                  c                          ,                          n                          ,                          k                                                                    -                      s                                        "RightBracketingBar"                                                  )                            ⁢                              "AutoLeftMatch"                }                                                    ,                            eq        .                  xe2x80x83                ⁢        1            
where
i=ADC sampling index (xe2x88x92I/2xe2x89xa6ixe2x89xa6I/2xe2x88x921);
n=azimuth position index (xe2x88x92N/2xe2x89xa6nxe2x89xa6N/2xe2x88x921);
k=IFSAR antenna phase center index (k=0,1 for single baseline IFSAR);
rc,n,k=vector from scene center to the effective phase center of the antenna;
s=vector from scene center to the target location;
fn,k=nominal center frequency for the sampled received pulse; and
Beff,n,k=effective bandwidth within the sampled data that determines range resolution.
Note that this formulation allows center frequency and bandwidth to vary as a function of both pulse number and antenna index. Furthermore, ADC sample times are chosen to track the scene center, that is, such that i=0 after a delay corresponding to the nominal range to the scene center 2|rc,n,k|/c.
For a target located with x-y-z coordinates (sx,sy, sz), we can expand
(|rc,n,k|xe2x88x92|rc,n,kxe2x88x92s|)≈sx cos "psgr"n,k sin xcex1n,kxe2x88x92sy cos "psgr"n,k cos xcex1n,k+sz sin "psgr"n,k
where "psgr" is a grazing angle from the respective antenna phase centers, and xcex1 is an azimuth angle.
While errors from this approximation need to be dealt with in high-performance IFSAR processing, this approximation is nevertheless adequate to explore motion compensation issues and other IFSAR features.
As shown in FIG. 2, within the Fourier space of the target scene, the video samples from a single pulse at a single antenna position describe a linear sequence of samples over a radial segment       (                  4        ⁢        π            c        )    ⁡      [                  f                  n          ,          k                    +                                    (                          B                              eff                ,                n                ,                k                                      )                    ⁢          i                I              ]  
at polar angles xcex1n,k and "psgr"n,k. Because the AC pulse is chirped (i.e., its frequency changes value with time), the target scene is represented as a straight line having a length indicative of the change in frequency. For a single antenna phase center in motion (fixed k), the collection of pulses is a series of generally parallel lines (displaced from each other by the movement of the antenna) which describe a collection surface in Fourier space.
Multiple antenna phase centers (multiple k) describe multiple collection surfaces (one for each antenna) in the same Fourier space. FIG. 3 shows the collection surfaces for the two vertically-arranged antennas of an IFSAR with broadside squint angle and constant waveform parameters. (Squint angle is the angle projected on the ground plane between the line of travel of the antenna and the line from the antenna to the target. Broadside squint means the line to the target it perpendicular to the direction of travel.) The two surfaces define a 3-dimensional volume that is effectively a 3-dimensional aperture in Fourier space.
To facilitate spatial coherence between the two images, each collection surface for prior art systems is typically cropped and resampled such that their projections onto the plane xcfx89z=0 is a common region,which ideally is rectangular. Note that the small bands at either end of the projection are not common regions, as only the upper surface contributes to the lower band and only the lower surface contributes to the upper band. The height information exists between the spaced collection surfaces, but accurate height information is difficult to determine if the surfaces are offset from one another, as shown, and common elements from each surface do not project to the xcfx89z=0
It has previously been proposed that a nearly common trapezoidal projection may be accomplished by pulse-to-pulse adjustment of center frequency in the manner fn,k=xcexan,kf0,0, where f0,0 is a nominal constant value, and       κ          n      ,      k        =                    cos        ⁢                  xe2x80x83                ⁢                  ψ                      0            ,            0                                      cos        ⁢                  xe2x80x83                ⁢                  ψ                      n            ,            k                          ⁢        cos        ⁢                  xe2x80x83                ⁢                  α                      n            ,            k                                .  
(See C. Jakowatz et al., Spotlight-Mode Synthetic Aperture Radar: A Signal Processing Approach, ISBN 0-7923-9677-4, Kluwer Academic Publishers, 1996.) Such adjustment improves the sitation, but does not resolve all errors.
It is an object of this invention to cause the collection surfaces to overlap to remove the need for cropping out some of the information.
It is another object of this invention to process height information earlier in the process to correct errors that otherwise distort the result.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a method of generating an IFSAR image of a target scene by transmitting and receiving a series of pulses from an IFSAR device having at least a pair of receiving antennas, the distance between said antenna phase centers being represented by a baseline projection, the method comprising:compensating for variations in vertical separation between collection surfaces defined for each antenna by adjusting the baseline projection during image generation. Furthermore, the invention comprises processing height information from all antennas before processing range and azimuth information in a normal fashion to create the IFSAR image.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.